Transfer-metallizing method

ABSTRACT

A transfer-metallization method in which a metal coating is applied to a top layer of a metallizing film, and the film is then brought into contact with a substrate such that the metal coating is transferred from the top layer to the substrate. The film is a three-layer biaxially-oriented transfer film produced by coextrusion and having a base layer of a polypropylene homopolymer and top layers applied to the two surfaces thereof. The base layer contains an additive depot of migrating monomers in an amount of 0.01 to 2.0 percent by weight, relative to the weight of the polymer constituting the base layer. The top layers are comprised of polypropylene that has a lower viscosity than the polypropylene of the base layer.

This application is a division of application Ser. No. 775,329, filedSept. 12, 1985, now U.S. Pat. No. 4,777,081.

BACKGROUND OF THE INVENTION

The present invention relates to a biaxially oriented transfermetallizing film based on polypropylene.

For certain packaging purposes, there exists a high, continuouslygrowing demand for composite materials of paper and metal, in particularaluminum foil. Although extremely thin metal foils are already beingused for such composite materials and satisfactory results are achievedusing these foils, attempts have been made to save even more of thescarce and costly metal by the use of metallized papers.

The state of the art has disclosed two different methods of metallizingpaper. In accordance with the first method, the metal is depositeddirectly on the paper under high vacuum, whereas according to the secondmethod, the metal is first vacuum-deposited on another substrate andthen transferred to the paper. The second process is also referred to astransfer metallizing.

For the direct vacuum-metallizing of paper, the choice of an appropriatepaper quality is a decisive factor; furthermore, an expensivepreparation of the paper is required. This preparation comprises sealingthe paper by means of a varnish layer or subjecting the paper to adrying step, during which the normal water content of paper, that is,about 6 to 7%, is reduced to about 3%. Following vacuum-metallizing, thepaper has to be readjusted to its normal water content. These proceduresinvolve high expenditures of cost and time.

For transfer metallizing, a plastic film, usually based on a polyolefinor polyester, is provided with a metal coating. The metallized film iscoated with a contact adhesive and bonded to the paper to be metallized,which in this case is selected in accordance with the requirements ofthe intended use. After the curing of the contact adhesive, the plasticfilm is peeled off from the composite structure, with the metal layerremaining on the paper. The process of transfer metallizing can beperformed economically only if the plastic film can be reused severaltimes. Conventional prior art films can be reused only up to about sixtimes, and thus, there is a need in the art for a plastic film which canbe reused to a greater extent.

SUMMARY OF THE INVENTION

It is therefore an object of the instant invention to provide a filmmade of a thermoplastic polymer, which for reasons of economy can bereused as often as possible in the transfer metallizing of paper.

It is another object of the invention to provide a film, as above, whichis multi-layered and biaxially oriented.

It is yet another object of the invention to provide a film, as above,in which the layers are coextruded.

It is yet another object of the invention to provide a process forpreparing a metallizing film.

These objects are achieved by a biaxially oriented three-layer transfermetallizing film, comprising a propylene homopolymer base layercontaining an additive depot of migrating monomers, the migratingmonomers present in an amount of from about 0.01 to about 2.0% by weightrelative to the weight of the propylene homopolymer, and a separate toplayer applied on each side of the base layer, each top layer constructedof polypropylene having a viscosity lower than that of the base layer.

The objects of the invention are further achieved by a process forproducing a biaxially oriented three-layer transfer metallizing film,comprising coextruding and biaxially orienting a propylene homopolymerbase layer with top layers of polypropylene having a viscosity lowerthan that of the base layer, one of the top layers positioned on eachside of the base layer, wherein the base layer contains an additivedepot of migrating monomers, present in an amount of from about 0.01 toabout 2.0% by weight relative to the weight of the propylenehomopolymer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides for a transfer metallizing film of the typedescribed above, which is characterized in that it is a three-layer filmproduced by co-extrusion. The film includes a base layer of propylenehomopolymer and a top layer applied to each surface of thepolypropylene. An additive depot of migrating monomers is contained inthe base layer in an amount of 0.01 to 2.0 percent by weight, relativeto the weight of the polymer forming the base layer. The top layers aremade of a polypropylene having a lower viscosity than the polypropyleneof the base layer.

The term "additive depot of migrating monomers" defines an admixture ofadditives in the base layer which includes compounds that, as a resultof diffusive forces, are capable of gradually penetrating from the baselayer into the top layers and of developing their action in the toplayers. The additives contained in the base layer in particular includea combination of stearic acid amide and a hydroxyethylalkylaminepossessing alkyl chains comprising 14 to 20 carbon atoms.

The quantity of additives contained in the base layer preferably amountsto between 0.01 and 1.0 percent by weight, in particular between 0.1 and0.5 percent by weight, relative to the weight of the polypropyleneforming the base layer.

The film according to this invention has a thickness ranging from 15 to30 μm and preferably from 20 to 25 μm, whereby each of the top layershas a thickness of between 0.3 and 5 μm, preferably of between 0.5 and1.5 μm.

Preferably, the top layers additionally contain inert admixtures whichimprove the slip properties or blocking resistance of the film.Admixtures of this kind include, for example, inorganic additives, suchas calcium carbonate, barium sulfate, silicon dioxide, aluminum silicateand similar substances, or combinations thereof. Special preference isthereby given to pellet-shaped aluminum silicate of a particle diameterof 0.2 to 0.8 μm. This helps in particular to avoid the formation ofscratches in the metal surface. Admixtures are contained in the toplayers in amounts of 0.1 to 1.0 percent by weight, relative to theweight of the top layers. Thereby, preference is given to aluminumsilicate, added in an amount of 0.2 to 0.6 percent by weight, relativeto the weight of the top layers.

The viscosities of the polymers forming the base layer and the toplayers, respectively, are indicated by means of their melt flow indices(MFI). The polymer of the base layer has a preferred MFI of 230/5 withina range of 5 to 20 g/10 min., and the polymer of the top layers has apreferred MFI of 230/5 within a range of 15 to 30 g/10 min., theviscosity of the polymer constituting the base layer being higher ineach case than the viscosity of the polymer constituting the top layers.

The film according to the present invention offers the particularadvantage that both of its surfaces can be used. After thevacuum-metallization of one of the film surfaces and transfer of themetal to the intended paper, the other surface of the film can bevacuum-metallized in a subsequent step, and so on. It has found to beparticularly advantageous in practical use to metallize the film on oneside prior to the first transfer metallizing step, in order to avoidelectrostatic charging during winding and then to perform transfermetallizing from the opposite side. Due to undesired electrostaticcharges, discharge structures form on the film surface, which during thetransfer metallizing are reproduced on the metal surface and lead toconsiderable quality reductions.

The metals for which the described film is suitable in particularinclude gold, silver, copper, nickel, tin and aluminum, and optionallyalso mixtures or alloys of at least two of these metals, aluminum beingparticularly suitable. The metals are usually deposited in layerthicknesses of 5 to 500 nm, depending on the type of support to whichthe metal layer is to be transferred. In general, the metal layers havethicknesses of 10 to 60 nm.

Processes by which the described metal layers can be applied are knownand include, for example, vacuum deposition, steam phase plating,current-free plating and electroplating, and are employed alone or incombination. In industrial processes, vacuum deposition is particularlypreferred.

EXAMPLE

A three-layer film comprising a base layer and two top layers wasproduced by coextrusion and biaxially oriented. The resulting film had atotal thickness of 25 μm, the thickness of the base layer being 23 μmand the thickness of each top layer being 1 μm. The base layer was apropylene homopolymer having an MFI 230/5 of 12 g/10 min. and containedthe following additives:

0.22% by weight of hydroxyethylalkylamine, with the alkyl group having18 carbon atoms; and

0.25% by weight of stearic acid amide.

The two top layers each consisted of a polypropylene having an MFI 230/5of 18 g/10 min., which additionally contained 0.4 percent by weight ofaluminum silicate.

The film could be used 20 times without any difficulties arising for thetransfer metallizing of paper which could be produced in a constantquality. Each time, the film was high-vacuum metallized with an aluminumlayer having a thickness of 40 nm.

It was only after repeating the treatment 20 times that small voidsappeared during the transfer of the metal layer to the paper, which weredue to a consumption of the additive depot in the base layer.

What is claimed is:
 1. A transfer-metallization method comprising thesteps of (a) applying a metal coating to a top layer of a metallizingfilm, then (b) bringing said film into contact with a substrate suchthat said coating is transferred from said top layer to said substrate,and (c) repeating steps (a) and (b) at least once such that said film isused on both sides in each repetition, wherein said metallizing film isa biaxially-oriented transfer metallizing film comprising:(a) apropylene homopolymer base layer; (b) at least one top layer applied oneach side of said base layer, said top layer consisting PG,10essentially of a propylene homopolymer and receiving a quantity of atleast one migrating compound from said base layer; (c) a first quantityof at least one compound capable of migrating from said base layer, saidfirst quantity being contained in said top layer and being at leastpartially depleted from said top layer by transfer of said metal layer,said compound comprising a mixture of stearic acid amide and ahydroxyethylalkylamine that comprises an alkyl group containing 14 to 30carbon atoms; (d) a second quantity of at least one migrating compoundcontained in said base layer, said second quantity being capable ofmigrating from said base layer to said top layer to replenish said firstquantity upon transfer of said metal layer, wherein said second quantityis comprised of a mixture of stearic acid amide and ahydroxyethylalkylamine that comprises an alkyl group containing 14 to 30carbon atoms.
 2. A method as claimed in claim 1, wherein said substrateis comprised of paper.
 3. A method as claimed in claim 1, wherein saidmixture contained in said base layer is present in an amount of fromabout 0.01 to about 2.0% by weight, relative to the weight of saidpropylene homopolymer in said base layer.
 4. A transfer-metallizationmethod, comprising the steps of (a) applying a metal coating to a toplayer of a metallizing film, then (b) bringing said film into contactwith a substrate such that said coating is transferred from said toplayer to said substrate, and (c) repeating steps (a) and (b) at leastonce such that said film is used on both sides in each repetition,wherein said metallizing film is a biaxially-oriented transfermetallizing film, comprising:a propylene homopolymer base layercontaining an additive depot of migrating monomers, said migratingmonomers (i) being present in an amount of from about 0.01 to about 2.0%by weight relative to the weight of said propylene homopolymer and (ii)comprising a mixture of stearic acid amide and a hydroxyethyalkylaminethat comprises an alkyl group containing 14 to 30 carbon atoms; and aseparate top layer applied on each side of said base layer, each saidtop layer consisting essentially of polypropylene having a viscositylower than that of said base layer, such that migrating monomers movefrom said base layer into at least one of said top layers.
 5. A methodas claimed in claim 4, wherein said substrate is comprised of paper. 6.A method as claimed in claim 4, wherein said mixture of stearic acidamide and hydroxyethylalkylamine is present in an amount of from about0.01 to about 2.0% by weight, relative to the weight of said propylenehomopolymer in said base layer.